The present invention relates to short-wavelength light generating apparatus using an external-cavity semiconductor laser (laser diode), which apparatus is applicable to a high-density optical disk system and others.
Recently, in optical disk high-density recording and image processing fields, there is a need to provide a green or blue light source by high-efficiency wavelength conversion using a semiconductor laser being used as an exciting (pumping) light source. The output light from the green or blue light source is required to be arranged to have the Gaussian transverse mode and to be converged up to near the diffraction limit and further required to be about several mW and stable in frequency and time. For obtaining such a high-power short-wavelength light generating apparatus using a semiconductor laser, a quasi-phase matching (QPM) type waveguide ("Optics Letters" Vol. 16, No. 15, 1156 (1991), written by Yamamoto and others) is used as a wavelength conversion means, or an intra-cavity (resonator) method is used which offers harmonic light with a semiconductor laser light being as the pumping light and a wavelength conversion device being provided within a resonator of a solid state laser.
FIG. 18 shows a schematic arrangement of a conventional short-wavelength light generating apparatus using a semiconductor laser and QPM polarization-inverted waveguide. In FIG. 18, light emitted from a semiconductor laser I01 is incident on a collimating lens I02 to be converted into a parallel light beam. The parallel light beam from the collimating lens I02 arrives at a half-wave (.lambda./2) plate I03 so that its deflection direction rotates, and then reaches a focusing lens I04, whose numerical aperture (NA) is 0.6, so as to be focused on an incident end surface I05 of a polarization-inverted waveguide I06, thereby obtaining blue light. Here, although anti-reflection coating is made on the end surface I05 of the waveguide I06 in order to prevent the return light to the semiconductor laser I01, in practice the return light of about 1% occurs.
FIG. 19 illustrates a schematic arrangement of an intra-cavity type short-wavelength light generating apparatus using a semiconductor laser pumped solid state laser. In FIG. 19, light emitted from a semiconductor laser Jo1 is incident on a collimating lens J02 to be converted into a parallel light beam and then incident on a focusing lens J03 to be focused on a laser material (Nd:YVO.sub.4) J04. An incident end surface J05 of the laser material J04 is coating-processed so as to allow anti-reflection (AR) with respect to the wavelength (809 nm) of the light from the semiconductor laser J01 and allow high reflection (HR) with respect to an oscillated wavelength (1.064 .mu.m) and harmonic light wavelength (532 nm). Further, the other surface J06 of the laser material J04 is antireflection-coated with respect to the oscillated wavelength and harmonic light wavelength. Light from the laser material J04 passes through a nonlinear optical material (crystal) KTP (KTiOPO.sub.4) J08 to reach an output mirror J07. The output mirror J07 is high-reflection-coated with respect to the oscillated wavelength. The output mirror J07 and the end surface J05 of the laser material J04 constitutes a resonator whose basic wavelength is 1.064 .mu.m so that the harmonic wave wavelength-converted by the nonlinear optical material KTP J08 can be obtained by the output mirror J07.
However, although the FIG. 18 apparatus can provide 1.1 mW blue light in relation to an incident light intensity 25 mW into the QPM polarization inverted waveguide, since the wavelength acceptance of the QPM polarization-inverted waveguide device is only 0.2 nm, the fluctuation of the oscillation frequency is 0.2 nm/.degree.C. in relation to the fluctuation of the temperature of the semiconductor laser and the mode hopping due to the return light appears to be about 1 nm, the output is stable only for several seconds. Thus, the FIG. 18 apparatus is required to be arranged to make stable the wavelength of the semiconductor laser.
Further, in the FIG. 19 apparatus, in the case that a single longitudinal mode semiconductor laser is used as a pumping laser, the return light occurring because the transmission factor of the anti-reflection coating on the end surface (Nd:YVO.sub.4) J05 of the laser material is about 93% causes the mode hopping and multiple longitudinal modes to occur, thereby generating noises because the half width of the absorption spectrum is several nm in the case of Nd:YVO.sub.4. Thus, the FIG. 19 apparatus is also required to be arranged to make stable the wavelength of the semiconductor laser.
In the above-described FIGS. 18 and 19 apparatus, as one countermeasure for the return light, it is considered to use an isolator or improve the anti-reflection coating on the incident end surface. The isolator cannot prevent the wavelength fluctuation due to the fluctuation of temperature of the semiconductor laser itself concurrently with being costly and large in size. Further, difficulty is encountered to improve the anti-reflection coating in accordance with the current technique.